F multi retrieve

Summary

Implements multi-entity retrieval caosdb/customers/lfpb/management#393

See tests in caosdb-cppinttest!4 (merged)

Focus

• Most relevant changes are in include/caosdb/transaction.h and src/caosdb/transaction.cpp.
• New class MultiResultSet represents a result set with multiple entities. Is this a good name?
• New enum TransactionType restricts the kind of sub-transactions (Insert, Delete, Retrieve) which can be added to a transaction.
• Macros are used to check whether InsertEntity, DeleteById or RetrieveById can be called on a transaction (given the TransactionStatus and 'TransactionType`).

Test Environment

Test with server in branch f-grpc-f-multi-retrieve

Check List for the Author

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• All automated tests pass
• Reference related Issues
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  • Intent of new code
  • Problems with old code
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Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
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include/caosdb/status_code.h

@@ -24,6 +24,8 @@
 
 #include <string>
 
+namespace caosdb {
+
 /**
  * StatusCodes represent the status of this client, it's connections,
  * configuration and so on.
@@ -33,9 +35,6 @@
  * GENERIC_TRANSACTION_ERROR indicates that *there are* errors in a
  * transaction).
  */
-
-namespace caosdb {
-
 enum StatusCode {
   INITIAL = -2,
   EXECUTING = -1,